Abstract: At approximately 3:00 p.m. on November 20, 1990, Employees #1 and #2 were changing the ethylene oxide (EtO) supply cylinders in the central supply room. The cylinders contained EtO/freon mixtures under pressure at a ratio of 12 to 88. The cylinders were connected through a valve to a supply line to another valve system, and then to the sterilization chamber. Employee #1 disconnected the supply line from an ethylene oxide cylinder without properly venting the system. Employees #1 and #2 heard hissing sounds coming from the lines, and the EtO gas leak tester that Employee #2 carried sounded a pulsating alarm when the dual set-point system went off, indicating more than 30 ppm EtO in the air. Employees #1 and #2 received medical treatment that day, and again two days later after experiencing shortness of breath, eye and upper respiratory system irritation, mouth blisters, and headaches. Employee #1 was off work for 2 days because of the incident. Employees #1 and #2 had not been given adequate training in the handling of ethylene oxide cylinders. A coworker who regularly changed the EtO cylinders was not available on the day of the accident.